1. Field of the Invention
The present invention relates to an output circuit in semiconductor integrated circuitry, and particularly, to an output circuit having an output transistor the emitter of which is grounded, the base of which serves as an input node for a control current and the collector of which serves as an output node.
2. Description of the Related Art
As a conventional example of an output circuit having an output transistor, there have been available, for example, an output circuit disclosed in JP-A 9-51256 (1997) or an output circuit shown in FIG. 8.
The circuit shown in FIG. 8 includes an output transistor Q1, other transistors Q2 to Q6, a current source I1 and a load X. To be detailed, the transistors Q2 and Q3 are connected between the emitters thereof, the base of the transistor Q2 serves as an input for an input signal VIN, the base of the transistor Q3 serves as an input for a reference voltage V1, and the current source I1 is connected between a power supply voltage VCC and the emitters of the transistors Q2 and Q3 in order to supply a current to the emitters thereof. The collector of the transistor Q4 is connected to the collector of the transistor Q2 and the emitter thereof is grounded, and the collector and base of the transistor Q5 are connected to the collector of the transistor Q3 and the base of the transistor Q4 and the emitter thereof is grounded. The collector of the output transistor Q1 is connected to an output VOUT, the base thereof is connected to a connecting point between the collector of the transistor Q2 and the collector of the transistor Q4 and the emitter thereof is grounded. The collector of the transistor Q6 is connected to the power supply voltage VCC, the base thereof is applied with a reference voltage V2 and the emitter thereof is connected to the output VOUT. The load X is connected between the output VOUT and the power supply voltage VCC.
Then, description will be given of operations in a conventional output circuit shown in FIG. 8. A current of the current source I1 is split into collector currents of the transistors Q2 and Q3 according to a voltage difference between the input signal VIN and a reference voltage V1. The collector current in the transistor Q3 becomes a collector current of the transistor Q4 by the action of a current mirror circuit constituted of the transistors Q4 and Q5. Since the collectors of the transistors Q2 and Q4 are connected to each other, a difference between the collector currents thereof is a base current of the transistor Q1 and a collector current hfe (an h parameter expressing a current amplification factor) times as large as the base current flows through the load X and the power supply voltage VCC drops by an inter-terminal voltage of the load X to obtain a potential at the output VOUT. In a case where the input signal VIN>the reference voltage V1, a collector current of the transistor Q4 is larger than that of the transistor Q2, so that a current is extracted from the base of the output transistor Q1 to reduce a current flowing into the load X and raise a voltage at the output VOUT. To the contrary, in a case where the input signal VIN<the reference voltage V1, a collector current of the transistor Q2 is larger than that of the transistor Q4, so that a base current flows into the base of the transistor Q1, a collector current hfe times as large as the base current flows through the load X to reduce a voltage at the output VOUT. In a case where the voltage at the output VOUT falls to a value lower than a voltage lower than the reference voltage V2 by VBE, the transistor Q6 acts to limit falling of a voltage at the output VOUT so as not to be lower than a voltage lower than the reference voltage V2 by VBE.
In the situation where a collector current of the output transistor Q1 increases, a voltage at the output VOUT falls and limitation is placed on falling of the voltage thereat by the transistor Q6, a current flowing through the collector of the transistor Q6, of a collector current of the output transistor Q1, for the limitation is useless since it does not contribute to a change in output voltage in response to the input signal VIN. Especially, in a case where a sharp change in an instant during which an output transitions is desired and the voltage after the change has only to be retained in value, a current necessary only for a period when an output voltage transitions continues to flow after the change in output voltage, having led to a fault to increase a consumption current.